96 report — 1877. 



however practised in the art of making observations, or however attentive he may 

 be. The difference between the time of a man's noting the event and that of its 

 actual occurrence is called his personal equation. It remains curiously constant in 

 every case for successive years, it is carefully ascertained for every assistant in every 

 observatory, it is published along with his observations, and is applied to them just 

 as a correction would be applied to measurements made by a foot-rule that was 

 known to be too long or too short by some definite amount. Therefore the magni- 

 tude of a man's personal equation indicates a very fundamental peculiarity of his 

 constitution ; and the inquiry I would suggest is to make a comparison of the age, 

 height, weight, colour of hair and eyes, and temperament (so far as it may admit of 

 definition) in each observer in the various observatories at home and abroad, with 

 the amount of his personal equation. We should thus learn how far the more 

 obvious physical characteristics may be correlated with certain mental ones, and 

 we should perhaps obtain a more precise scale of temperaments than we have at 

 present. 



Another subject of exact measurement is the time occupied in forming an ele- 

 mentary judgment. If a simple signal be suddenly shown, and if the observer 

 presses a stop as quickly as he can when he sees it, some little time will certainly 

 be lost, owing to delay in nerve-transmission and to the sluggishness of the me- 

 chanical apparatus. In making experiments on the rate of judgment, the amount 

 of this interval is first ascertained. Then the observer prepares himself for the 

 exhibition of a signal that may be either black or white, but he is left ignorant 

 which of the two it will be. Ho is to press a stop with his right hand in the first 

 event, and another stop with his left hand in the second one. The trial is then 

 made, and a much longer interval is found to have elapsed between the exhibition 

 of the alternative signal and the record of it than had elapsed when a simple signal 

 was used. There has been hesitation and delay : in short, the simplest act of judg- 

 ment is found to consume a definite time. It is obvious that here, again, we have 

 means of ascertaining differences in the rapidity of forming elementary judgments 

 and of classifying individuals accordingly. 



It would be easy to pursue the subject of the measurement of mental qualitii s 

 to considerable length, by describing other kinds of experiment, for they are numer- 

 ous and varied. Among these is the plan of Professor Jevons, of suddenly exhi- 

 biting an unknown number of beans in a box, and requiring an estimate of their 

 number to be immediately called out. A comparison of the estimate with the fact, 

 in a large number of trials, brought out a very interesting scale of the accuracy of 

 such estimates, which would of course vary in different individuals, and might bo 

 used as a means of classification. I can imagine few greater services to Anthro- 

 pology than the collection of the various experiments that have been imagined to 

 reduce the faculties of the mind to exact measurement. They have engaged the 

 attention of the highest philosophers, but have never, so far as I am aware, been 

 brought compendiously together, and have certainly not been introduced, as they 

 deserve, to general notice. 



Wherever we are able to perceive differences by intercomparison, we may rea- 

 sonably hope that we may at some future time succeed in submitting those differ- 

 ences to measurement. The history of science is the history of such triumphs. I 

 will ask your attention to a very notable instance of this, namely, that of the esta- 

 blishment of the scale of the thermometer. You are aware that the possibility of 

 making a standard thermometric scale wholly depends upon that of determining 

 two fixed points of temperature, the interval between them being graduated into a 

 scale of equal parts. These points are, I need hardly say, the temperatures of freezing 

 and of boiling water respectively. On this basis we are able to record temperature 

 with minute accuracy, and the power of doing so has been one of the most important 

 aids to Physics and Chemistry as well as to other branches of investigation. We 

 have been so accustomed from our childhood to hear of degrees of temperature, 

 and our scientific knowledge is so largely based upon exact thermometric measure- 

 ment, that we cannot easily realize the state of science when the thermometer, 

 as we now use it, was unknown, Yet such was the condition of affairs so recently 



